1. Field of the Invention
This invention relates to a fuel injection amount control system for controlling an amount of fuel injected into an intake passage of an internal combustion engine, and an intake passage wall temperature-estimating device for use with the control system, and more particularly to a fuel injection amount control system of this kind which is adapted to correct the fuel injection amount so as to compensate for delay in transfer of part of injected fuel to combustion chambers of the engine, and an intake passage wall temperature-estimating device for use with the control system.
2. Prior Art
While part of fuel injected via fuel injection valves into an intake pipe of an internal combustion engine directly flows into a combustion chamber of the engine, the remainder thereof once adheres to wall surfaces of the intake pipe including intake ports and then carried off the wall surfaces after a while to flow into the combustion chamber. A fuel injection amount control system is conventionally known, which estimates an amount of fuel to adhere to wall surfaces and an amount of fuel to be carried off the adherent fuel into the combustion chamber due to evaporation and other factors, and then determines an appropriate amount of fuel to be injected (fuel injection amount), by taking into account these estimated amounts of fuel, i.e. by effecting fuel transfer delay-dependent correction of the fuel injection amount.
The amount of fuel adhering to the wall surfaces of the intake pipe (hereinafter referred to as "the adherent fuel amount") is estimated based on a direct supply ratio A defined as the ratio of an amount of fuel directly drawn into a combustion chamber of a cylinder in one cycle of the cylinder to an amount of fuel injected for the cylinder in the same cycle, and a carry-off supply ratio B defined as the ratio of an amount of fuel carried off fuel adhering to the wall surfaces of the intake pipe into the combustion chamber of the cylinder through evaporation and other factors to an amount of the fuel adhering to the wall surfaces. An amount of fuel carried off the adherent fuel (hereinafter referred to as "the carried-off fuel amount") is estimated based on the carry-off supply ratio B and the adherent fuel amount.
More specifically, assuming that the adherent fuel amount is represented by Fw, the carried-off fuel amount by Fwout, and the fuel injection amount by Tout, a required fuel amount Tcyl, i.e. an amount of fuel required by the cylinder can be expressed by the following equation: EQU Tcyl=A.times.Tout+Fwout EQU where Fwout=B.times.Fw
Therefore, the fuel injection amount Tout can be expressed as follows: EQU Tout=(Tcyl-Fwout).times.(1/A)
However, such a fuel transfer delay-dependent correction is not sufficient for ensuring that the air-fuel ratio of a mixture supplied to the engine is properly controlled to a desired air-fuel ratio. For example, if fuel injection valves employed in the engine have operating characteristics other than proper ones, or a reference pressure set to a pressure regulator of a fuel pump of the engine deviates from a proper level, there arises an error in the actual fuel injection amount even if the fuel injection valve is driven by a pulse having an accurate pulse width. Similarly, variations in charging efficiency between individual engines (the charging efficiency determines an amount of fuel drawn into combustion chambers of the engine) can result in an unsuitable value of fuel injection amount which is set from a basic fuel injection amount map according to the engine rotational speed and pressure within the intake pipe, resulting in an error in the fuel injection amount Tout.
To eliminate such an error of the fuel injection amount ascribed to errors on the fuel injection valve side or manufacturing tolerances and/or aging of the engine, it has been conventionally proposed to carry out fuel transfer delay-dependent correction of the fuel injection amount by the use of an air-fuel ratio correction coefficient KO2 which is used in air-fuel ratio feedback control responsive to an output from an oxygen concentration sensor arranged in the exhaust system of the engine and which includes correction terms for correction of the above errors and tolerances, etc.
One of the proposed methods (first method) is disclosed by Japanese Provisional Patent Publication (Kokai) No. 58-8238 (corresponding to Japanese Patent Publication (Kokoku) No. 3-59255) in which the fuel injection amount Tout is obtained by multiplying the required injection amount Tcyl by the correction coefficient KO2 as expressed by the following equation: EQU Tout=(Tcyl.times.KO2-Fwout).times.(1/A)
Another method (second method) is disclosed by Japanese Provisional Patent Publication (Kokai) No. 61-126337, in which a Tout value corrected for the adherent fuel is multiplied by the correction coefficient KO2 to obtain the fuel injection amount Tout by the use of the following equation: EQU Tout=[(Tcyl-Fwout)/A].times.KO2
According to the O2 feedback control using the correction coefficient KO2, the air-fuel ratio correction coefficient KO2 is calculated based on an output from an air-fuel ratio sensor (oxygen concentration sensor) arranged at a location upstream of a catalytic converter arranged in an exhaust passage of the engine, and the fuel injection amount Tout is determined based on the air-fuel ratio correction coefficient KO2.
However, the first and second methods suffer from the following problems:
(1) The correction of errors in the operating characteristics of fuel injection valves should be carried out such that the operating characteristics of the fuel injection valves alone are corrected without correcting a real or physical amount (g) of fuel injected thereby.
More specifically, let it be assumed that a fuel amount required by the engine is 10 g, and delivery of an injection pulse having a pulse width of 20 ms has been hitherto sufficient or suitable for injecting 10 g of fuel. If the fuel injection valve is replaced by one having a reduced nozzle bore, an injection pulse having a pulse width of 22 ms should be delivered to the fuel injection valve so as to adapt the operation of the fuel injection valve to the fuel amount required by the engine. In this case, although the injection pulse width is increased from 20 ms to 22 ms, the real or physical amount of fuel injected remains equal to 10 g.
Thus, in correcting the errors on the fuel injection valve side, it is not required to correct the real or physical amount (g) of fuel injected, but it suffices to correct only the width of an injection pulse supplied to the fuel injection valve. When the fuel injection valve is replaced by one having a reduced nozzle bore as in the above example, the value of the correction coefficient KO2 is increased accordingly, so that the injection pulse width is increased. However, the real or physical amount (g) of fuel flowing into the cylinder remains unchanged. Therefore, it is not required to increase the carried-off fuel amount Fwout (i.e. reduce the adherent fuel amount) as an amount of fuel carried off the fuel adherent to the wall surfaces of the intake pipe into the cylinder so as to follow up an increase in the KO2 value.
However, in the first method, anapparent or nominal amount of fuel (g) of Tcyl.times.KO2 is corrected as if this amount of fuel actually flowed into the cylinder, and hence if the fuel injection valve is replaced by one having a reduced nozzle bore as in the above example, the fuel injection amount Tout increased by the KO2 value (in the above example, by 10%) will be be reflected in the carried-off fuel amount Fwout after a certain time delay, resulting in an increase of 10% in the carried-off fuel amount. Thus, the correction of errors of operating characteristics of fuel injection valves by the first method causes the carried-off fuel amount Fwout to be unnecessarily changed following a change in the KO2 value, which prevents the fuel injection amount from being accurately corrected for fuel transfer delay.
In the second method as well, the fuel injection amount is apparently or nominally corrected such that an amount (g) of fuel multiplied by KO2 is injected, so that the carried-off fuel amount Fwout is changed in the same manner as in the first method, following the fuel injection amount Tout corrected by the KO2 value, which also prevents the fuel injection amount from being accurately corrected for fuel transfer delay.
(2) According to the air-fuel ratio control using the air-fuel ratio sensor (oxygen concentration sensor), the fuel injection amount Tout is increased or decreased by a change in the air-fuel ratio correction coefficient KO2 based on the output from the air-fuel ratio sensor. The air-fuel ratio correction coefficient KO2 is, therefore, a feedback control amount which increases and decreases cyclically with a varying repetition period. On the other hand, in the fuel transfer delay-dependent correction, the fuel injection amount Tout is corrected during a fuel transfer delay cycle, i.e. a change in the fuel injection amount.fwdarw.a change in the adherent fuel amount Fw.fwdarw.a change in the carried-off fuel amount Fwout. Thus, the carried-off fuel amount Fwout varies with a repetition period ascribed to this fuel transfer delay cycle. If the repetition period of change of the air-fuel ratio correction coefficient KO2 and the repetition period of change of the carried-off fuel amount Fwout become synchronous to each other, hunting of the KO2 value occurs, which prevents the fuel injection amount Tout from being properly determined.
For example, during a steady operating condition of the engine, e.g. when a vehicle with the engine installed therein is cruising, the intake pipe negative pressure and the engine rotational speed are nearly constant, so that the direct supply ratio A and the carry-off supply ratio B remain unchanged, with the required fuel amount Tcyl maintained constant. Even on such an occasion, according to the first and second methods, if the KO2 value is changed such that the air-fuel ratio of the mixture is converged to a desired air-fuel ratio, the fuel injection amount Tout is changed accordingly. The change in the fuel injection amount Tout is fed back to cause a change in the KO2 value with a time lag and hence changes in the fuel injection amount Tout and the carried-off fuel amount Fwout. Therefore, if the repetition period of change of the KO2 value and the period of change of the carried-off fuel amount Fwout become synchronous to each other, there occurs hunting of the KO2 value across the desired air-fuel ratio due to an excessive correction effected by the synchronous combination of the air-fuel ratio feedback control and the fuel transfer delay-dependent correction of the fuel injection amount.
As a result, the first and second methods conventionally proposed suffer from the problem of degraded drivability and degraded exhaust emission characteristics of the engine.
Further, conventional fuel injection amount control systems including ones employing the first and second methods do not contemplate the fact that part of fuel supplied into the combustion chamber is not burnt in the cylinder (unburnt fuel), and hence suffer from the following problems:
As already stated above, although part of fuel injected from the fuel injection valves flows directly into the cylinder, and the remainder thereof once adheres to wall surfaces of the intake port and then carried off into the cylinder, all the injected fuel is supplied to the cylinder after all. However, part of the fuel drawn into the cylinder forms unburnt fuel, such as non-atomized fuel (liquid granules) and adherent fuel adhering to inner wall surfaces of the cylinder, which is often generated when the engine is started in a cold condition, or after fuel cut after the engine has been shifted from a cranking mode to a normal mode.
Unless the fuel injection amount is corrected for the unburnt fuel component (HC), it can occur that the air-fuel ratio (A/F) within the cylinder is leaner than a required value which actually contributes to combustion, and consequently the engine suffers from unstable combustion when it is in an operating condition where the unburnt fuel component (HC) is generated in large amounts, such as at the start of the engine and immediately after the start of the engine.
Further, some of the conventional fuel injection amount control systems have proposed to effect the fuel transfer delay-dependent correction of the fuel injection amount by taking into account the wall temperature of the intake port, in view of the fact that the adherent fuel amount depends not only on the intake pipe negative pressure and the engine rotational speed but also on the intake port wall temperature. In this connection, to avoid an increased cost ascribed to an increased number of component parts, it has been proposed to estimate the intake port temperature by calculation without using a wall temperature sensor for directly detecting the intake port temperature, e.g. by Japanese Patent Publication (Kokoku) No. 60-50974 (third method) and Japanese Provisional Patent Publication (Kokai) No. 1-305142 (fourth method).
The third method calculates or estimates the intake port wall temperature based on the engine coolant temperature, a cumulative value of the engine rotational speed counted up from the start of the engine, etc. Then, a basic fuel injection amount is determined based on the engine rotational speed and the intake air amount, and the value of the basic fuel injection amount thus obtained is averaged to obtain an averaged function value. Thereafter, a value of the difference between the value of the basic fuel injection amount and the averaged function value is determined, and then a fuel correction amount is determined based on the determined difference and the intake port wall temperature estimated. The resulting correction fuel amount is added to the basic fuel injection amount to determine the fuel injection amount.
The fourth method determines an equilibrium wall temperature assumed when fuel adhering to the wall surfaces of the intake port is in an equilibrium state, and a delay time constant representing a delay time of change of the intake port wall temperature, based on the intake pipe negative pressure and the engine rotational speed, and the equilibrium wall temperature is corrected by the engine coolant temperature and the intake air temperature to set an instant wall temperature. The instant wall temperature is subjected to a first order delay processing by the use of the delay time constant to determine an estimated intake port wall temperature for correction of the fuel injection amount.
According to the third and fourth methods, however, the behavior or characteristic of the intake port wall temperature is not accurately grasped, and hence the intake wall port temperature cannot be accurately estimated under all operating conditions of the engine. As a result, there still remains the problem that the fuel transfer delay-dependent correction of fuel injection amount cannot be effected accurately, based on the intake port wall temperature estimated by the conventional methods.